1. Field of the Invention
The present invention relates to a printed circuit board on which a flat package IC (Integrated Circuit) is mounted using a flow soldering technique.
2. Description of the Related Art
In recent years, there has been a trend to mount a greater number of components on a printed circuit board. Thus, mounting ICs (Integrated Circuits) with a flat package such as a SOP (Small Outline Package) or a QFP (Quad Flat Package) at small intervals is to be performed. On the other hand, to achieve a reduction in cost, mounting IC packages by flow soldering is to be performed. In the flow soldering, to achieve high performance in soldering without having solder bridges, the soldering process is to be controlled precisely.
A known technique to prevent formation of solder bridges is to form a dummy land on a downstream side of an IC as viewed in a solder flow direction. Hereinafter, the downstream side as viewed in the solder flow direction will be referred to simply as the downstream side. Another known technique is to increase the size of a most-downstream-located land of an IC such that this land serves as a solder sink land. FIG. 14 illustrates an example of a solder bridge prevention land for a SOP package. In FIG. 14, reference numerals 104 and 102 denote lands (copper-exposed parts of patterns) of an IC 103 with a package of the SOP type. Note that a no-resist part of a pattern where copper is exposed will be simply referred to as a copper-exposed part. The lands 102 are located at the most downstream positions of the IC 103 as viewed in the solder flow direction. In FIG. 14, the direction of the movement of the printed circuit board during the soldering process is represented by an arrow. That is, the solder flows in a direction opposite to the arrow and thus upstream and downstream sides of the solder flow are as shown in FIG. 14. In FIG. 14, thin solid lines represent pins of the IC 103. Note that these thin solid lines partially overlap thick lines representing the lands 102 and 104.
A solder bridge prevention land (also referred to as a dummy land) 101 is formed on the downstream side of the land 102 as viewed in the solder flow direction. In the flow soldering process, solder is drawn into the solder bridge prevention lands 101. This leads to a reduction in surface tension and interfacial tension of solder adhering to the lands 104 and 102 and the pins of the IC 103 located on the upstream side, and thus formation of solder bridges at the lands 104 and 102 and the pins of the IC 103 is prevented.
FIG. 15 illustrates another example of a solder bridge prevention land for a SOP package. The example shown in FIG. 15 is different from that shown in FIG. 14 in that a solder bridge prevention land 111 is formed by expanding a land to be connected to a pin 112, at the most downstream location, of an IC 103.
There are further known techniques to prevent formation of solder bridges as described below.
For example, Japanese Patent Laid-Open No. 63-213994 discloses a board pattern including lands on which to mount an IC with a package of the QFP type in a slant direction with respect to a solder flow direction and also including a dummy land formed on a downstream side in a solder flow direction thereby preventing formation of a solder bridge. Japanese Patent Laid-Open No. 2-119295 discloses a board pattern including lands for soldering a chip component and also including a dummy land formed on a downstream side in a solder flow direction thereby preventing formation of a solder bridge. Japanese Patent Laid-Open No. 4-208594 discloses a board pattern including lands for soldering an IC with a SOP-type package and also including a dummy land formed on a downstream side in a solder flow direction thereby preventing formation of a solder bridge. Japanese Patent Laid-Open No. 5-315733 discloses a board pattern for mounting an IC with a QFP-type package, including a solder sink land having a slit for well draining solder thereby preventing formation of a solder bridge. However, the conventional board patterns described above may have issues. More specifically, for example, use of a land for preventing formation of a solder bridge, such as a dummy land or a solder sink land, leads to a reduction in flexibility in designing a pattern of a printed circuit board. An example of low flexibility in designing patterns is a restriction on the size of a heat radiation pattern, which results in a reduction in heat radiation performance of a mounted IC. To avoid such a reduction in heat radiation performance, the size of a printed circuit board is to be increased so that a heat radiation pattern with a large enough size can be formed thereon, or a heat radiation plate in addition to the heat radiation pattern is to be provided. In a case where a printed circuit board needs a GND (ground) pattern having a large enough size and having a large enough number of connections to reduce an effect of a spurious radiation noise from a CPU (Central Processing Unit) or the like, the low flexibility in designing patterns leads to a restriction on the size of the ground pattern or the number of connections.
For example, in the case of a printed circuit board on which an IC having a heat radiation pin, such as a motor driver IC, is mounted, the existence of a land for preventing formation of a solder bridge such as a dummy land or a solder sink land can be an obstacle that makes it difficult to form a large-size heat-radiation copper film pattern connected to the heat radiation pin. That is, if the land for preventing formation of solder bridges is formed to be large enough, the result is a reduction in a space usable to form a heat radiation pattern. In the case of a printed circuit board having a signal line extending between a heat radiation pin and a heat radiation copper pattern, the existence of the signal line makes it difficult to directly connect the heat radiation pin to the heat radiation copper pattern. One method to realize the connection between them is to use a jumper wire to connect the heat radiation pin and the heat radiation copper pattern. However, formation of lands for the jumper wire results in a reduction in the size of the heat radiation copper pattern.
In recent years, there has been a trend toward a reduction in the pin-to-pin distance of an IC that is mounted via a flow soldering technique on a printed circuit board (and thus a reduction in the land-to-land distance of lands formed on the printed circuit board). To effectively prevent formation of solder bridges on such a printed circuit board, the size of a land for preventing formation of solder bridges is to be increased.